1. Field of the Invention
This invention is generally directed to photodetector arrays, and, more particularly to a compact system for blocking dark current and passing signal current by capacitive coupling.
2. Description of the Related Art
One type of electro-optic system using light as a signal carrier includes sensors of various parameters such as position, temperature, and pressure; fiber optic interconnects; and effectors such as actuators and relays. The advantages of an electro-optic system over an electrical system include reduced weight, reduced EMI (electro-magnetic interference) sensitivity, and reduced costs.
Multichannel spectrographs are one way of providing an optical-to-electronic conversion of the signals generated from optical sensors, especially those sensors that are used in wavelength division multiplexing networks or rely on wavelength encoding of the sensed parameter. Previous multi-channel spectrographs have used off-the-shelf charge coupled device (CCD) arrays to convert the light spectrum to an electronic signal. Most existing CCD structures are a poor match to the dispersed optical image from a spectrograph. Typically, the dispersed image has a height of about 130 .mu.m, whereas the CCD row height is about 13 .mu.m. Ninety percent of the available illumination is thus lost unless the signals from a number of imager rows are read out, converted to digital form, stored, and combined. This is an unwieldy, expensive approach.
In some applications, various components in electro-optical systems must operate at temperatures as high as 125.degree. C. Conventional silicon sensors such as photodiodes, charge injection devices, and charge coupled devices exhibit large dark currents at high temperatures. Although a custom CCD with the proper pixel height could be developed to reduce illumination loss, that CCD would not address the dark current problem. In some cases these dark currents saturate the photodetector and thus render the photodetector useless. In other cases, photodetectors that can handle the dark currents exhibit a spatial variation of dark current among the various photodetectors of an array which generates a high level of background signal level as compared with the desired signal level.
Available sensor arrays measure the total photodetector current, i.e., the sum of the dark current and signal current. Charge storage capacity is limited by the structure used and forces operation at short integration times with a resultant high frequency analog-to-digital conversion and ancillary electronics operation. Dark current varies from element-to-element, so that the background level is a pattern noise that is exponentially temperature dependent, and operation of sensor arrays is typically limited to a maximum temperature of 85.degree. C.